Mercaptocarboxylic acid (mercaptopropionic acid) is used for synthetic resins because it is suitable as a crosslinking agent of acrylic acid ester polymers and a curing agent of epoxy resins. In particular, a variety of carboxylic acid ester compounds can be synthesized by esterification reaction of such mercaptocarboxylic acid (mercaptopropionic acid) with alcohols.
U.S. Pat. No. 5,008,432 discloses preparation of 3-mercaptopropionic acid by adding hydrogen sulfide to an unsaturated compound such as methacrylate or acrylic acid. Addition of the hydrogen sulfide is carried out in the presence of a basic catalyst selected from magnesium oxide and an anion exchange resin. The resin was selected from those containing tertiary amines as functional groups and those containing quaternary ammonium hydroxides as functional groups. However, this method involves adding hydrogen sulfide to acrylic acid to convert the acrylic acid into 3-mercaptopropionic acid (HSCH2CH2COOH), thus making the process very complicated.
Korean Patent Laid-open No. 1998-024803 discloses a method for synthesizing 3-mercaptopropionic acid by addition reaction between acrylic acid and hydrogen sulfide. Like U.S. Pat. No. 5,008,432, the reaction is carried out in the presence of a solid support having a guanidine functional group which does not contain a hydrogen directly bonded to a nitrogen atom, instead of using amine.
Korean Patent Laid-open No. 1998-024803 discloses synthesis of 3-mercaptopropionic acid at a high conversion proportion and high selectivity through addition reaction with acrylic acid and hydrogen sulfide in the presence of a solid support having a basic functional group. However, the drawback of complicated preparation process remains.
CN Patent No. 101125827A discloses a method for converting thiodipropionic acid (dimer) into 3-mercaptopropionate (polymer). This method requires relatively vigorous reaction conditions due to acid treatment with strong acid and iron, and thus has problems of undesired composition of products and occurrence of by-products under such conditions.
In addition, Korean Patent No. 10-0350658 discloses a preparation method to easily treat 3-mercaptopropionitrile and 3-mercaptopropionic acid. The method of the present invention includes reacting with alkali hydrosulfide (alkali hydrogen sulfide) in the presence of an alkali hydroxide using thiodipropionitrile as a starting material to produce 3-mercaptopropionitrile at a high yield. The resulting nitrile is acidized with strong acid or saponified to obtain the desired mercaptopropionic acid at a high yield. With this invention, 3-mercaptopropionitrile and 3-mercaptopropionic acid can be obtained at high yields without incorporating dithiodipropionitrile and dithiodipropionic acid. However, this method should include preparing thiodipropionitrile (dimer) using acrylonitrile and sodium hydrosulfide, adding sodium hydrosulfide and sodium hydroxide thereto to prepare 2-sodium cyanoethanethiolate (monomer), and refluxing the 2-sodium cyanoethanethiolate in the presence of strong acid (hydrochloric acid) to convert nitrile into a carboxylic group. This causes many problems of complicated reaction step, difficulty in preparation, low yield of products resulting from conversion of the nitrile into the carboxylic group under the condition of refluxed hydrochloric acid, without separating 3-mercaptopropionitrile, and a great amount of 3-mercaptopropionitrile remains unreacted.
In order to solve these conventional problems, Korean Patent Laid-open No. 10-2013-0087447 provides a method for preparing 3-mercaptopropionic acid that eliminates the necessity of the process of obtaining 3-mercaptopropionitrile from thiodipropionitrile using thiodipropionitrile (dimer) with sodium hydrosulfide and sodium hydroxide. In accordance with this method, acrylonitrile is reacted with sodium hydrosulfide to obtain 2-sodium cyanoethanethiolate, the 2-sodium cyanoethanethiolate is neutralized with an acid, layer-separation is conducted to obtain 3-mercaptopropionitrile, the 3-mercaptopropionitrile is converted into 3-mercaptopropionic acid by addition of an acid and reflux, and the 3-mercaptopropionic acid is distilled under reduced pressure to obtain 3-mercaptopropionic acid. This method is a simple and economically efficient process, in that 3-mercaptopropionitrile involved in preparation of less dimer is obtained, and provides a higher yield compared to the aforementioned other methods. However, this method entails considerable yield loss and degradation of products resulting from high-temperature treatment in the process of obtaining 3-mercaptopropionitrile, and converting 3-mercaptopropionitrile into 3-mercaptopropionic acid by acid addition and distilling under reduced pressure. Therefore, there is a need for improvement that can enhance a yield while distilling products under reduced pressure at a lower temperature.